A method and a device for controlling a final controlling element in closed-loop are described, for example, in German Patent No. DE 41 05 740 (U.S. Pat. No. 5,085,190). A method and a device are described therein for controlling a controlling element in closed loop to influence the start of injection of a high-pressure fuel pump.
On the basis of a system deviation, an automatic controller defines a driving signal to be fed to the final controlling element. The output signal from the automatic controller is superposed with an output signal from a precontrol (pilot control), the precontrol exhibiting a response characteristic that is the inverse of the response characteristic of the final controlling element. To be able to compensate for tolerances between individual final controlling elements and for manifestations of ageing in an ongoing operation, the driving signal of the final controlling element is adapted.
The design of the automatic controller is problematic, in particular when the final controlling element exhibits integral action. This means that even when no driving signal for the final controlling element is at hand, the variable to be adjusted changes. For this reason, even in the absence of a system deviation, it is necessary that the final controlling element receive a driving signal that will not produce a change in the variable to be adjusted.
Furthermore, it is problematic to optimize the requirements with respect to dynamic response and control precision. To be able to achieve a closed-loop control with a good dynamic response, the automatic controller should essentially exhibit only a PD (proportional-plus-derivative) action. An integral-action component leads to degradation of the dynamic response. However, this integral-action component does improve the control precision.